Radio frequency (RF) transmitters are found in many one-way and two-way communication devices, such as portable communication devices, cellular telephones, personal digital assistants (PDAs) and other communication devices. An RF transmitter transmits using whatever communication methodology is dictated by the particular communication system within which it is operating. RF transmitters are often incorporated into devices that support transmission of voice or data at multiple frequency bands. Such RF transmitters typically employ an upconversion process to control the transmit frequency of the device. In general, the upconversion process involves super-imposing baseband information on a high frequency carrier signal.
Typically, an upconverter uses one or more mixers to upconvert a baseband signal or an intermediate frequency signal (e.g., a local oscillator (LO) signal) to an RF signal. A considerable problem with a conventional mixer used in an upconverter is that it generates undesirable noise. To suppress the noise, a local oscillator (LO) signal may be hard-switched or a square wave may be applied instead of a sinusoidal signal. As a result, the output signal may contain the modulated signal around the intermediate frequency (ωLO) and unwanted harmonics at odd harmonics, such as, third, fifth, and seventh harmonics. These harmonics create unwanted spurious power that may, for example, violate transmit mask requirements or may cause distortions in the transmitted output signal when they pass through output buffers or power amplifiers that may have non-linear operational characteristics. Therefore, it is desirable to remove these harmonics.
In a conventional mixer, passing ωLO but rejecting the odd harmonics (e.g., 3ωLO, 5ωLO) typically requires a relatively high-Q filter. The filter requirements may be simplified by using a harmonic rejection mixer. A typical third harmonic rejection mixer obtains four signals (A0°, A60°, A90°, A150°) by squaring, or passing through a limiter, sinusoidal signals at the local oscillator (LO) frequency with phase differences of 0, 60, 90, and 150 degrees with respect to each other. The sinusoidal signals are usually generated by using poly-phase filters that may include resistive-capacitive (RC) sections that realize phase shift. However, poly-phase filters are bulky, narrow-band, and consume a lot of power.
Thus, there is a need for improved systems and methods for implementing a harmonic rejection transmitter.